Monitoring optically powered actuators

ABSTRACT

Operation of a power-by-light valve ( 15 ) by a source ( 11 ) along an optical fibre ( 13 ) is monitored by means of optical sensors ( 14 ) from which light passes back along fibre ( 13 ) to monitoring and processing units ( 16, 17 ). The sensors ( 14 ) include fibre bragg gratings ( 41, 43 ) for detecting differential pressure across diaphragms ( 42 ) and for detecting temperature. At the input end, an optical coupler ( 12 ) supplies sensor signals to units ( 16, 17 ) which can control source ( 11 ) via a signal ( 32 ).

[0001] The present invention relates to systems incorporating anoptically powered actuator for example within control-by-light andpower-by-light valves.

[0002] Generally, there are three techniques which have been used toconvert optical power to operate a valve, which depend on the actuatortype.

[0003] 1. Convert optical power to electrical power by using aphotovoltaic device or photo diode to drive the actuator in the valve,preferably a ferroelectric actuator.

[0004] 2. Convert optical power to heat energy; this technique can beused with a valve using a temperature operated actuator such as abimetallic actuator, thermo-mechanical actuator or shape memory alloy.

[0005] 3. Use the optical power to operate the actuator in the valvedirectly, for example a valve which uses a photosensitive materialactuator.

[0006] U.S. Pat. No. 5,709,245 discloses an optically controlledactuator employing a laser, an optical fibre and a photocell whichenergises coils of an electro-hydraulic valve.

[0007] The present invention seeks to provide an opticallycontrolled/powered actuator having a built-in sensing arrangement. Thepresent invention further seeks to provide an arrangement for monitoringthe output of the integrated sensor.

[0008] According to a first aspect of the present invention there isprovided a monitoring system for an optically powered and/or controlledactuator arrangement, comprising an optical path arranged to beconnected between at least one optical source and the actuatorarrangement for supplying power and/or control information thereto, theactuator arrangement including means for converting the optical powerand/or control information characterised in that the actuatorarrangement further comprises means for sensing the operation thereofand providing an optical data output back through said optical path, andin that means are connected to the optical path to monitor the output ofthe sensing means.

[0009] In a preferred arrangement, the sensing means is also operated bysaid optical source.

[0010] In a preferred system the sensing means comprises one or morefibre Bragg gratings. The differential pressure of the actuator systemmay be sensed by an arrangement incorporating a fibre Bragg gratinglocated adjacent to at least one diaphragm. Preferably, the fibre Bragggrating is mounted between two diaphragms arranged to be exposed todifferent pressures. In addition, or instead, a fibre Bragg grating maybe provided for sensing the temperature; this grating may be used tocompensate for unwanted temperature effects on the differential pressuregrating.

[0011] The monitoring system may comprise at least two optical sources,a first of which is used for supplying power and/or control information,and a second of which is used to detect interruption of the optical pathand is arranged to turn off the first source in the event of suchinterruption.

[0012] In preferred systems, the output of the sources is controlled bya signal from the monitoring means. The use of such a feedback loopenables control of the actuator.

[0013] Preferably, the actuator arrangement is at a location remote fromthe optical source. Such an arrangement has the advantage that thecomponents at the end of the optical fibre remote from the source can berelatively simple and robust and have no local power requirements.

[0014] According to a second aspect of the present invention, there isprovided a method of using one or more light sources at a first end ofan optical path for powering and/or controlling a valve or actuator at asecond end of the optical path characterised in that light from thesource(s) is used to interact with one or more sensors, at or adjacentthe second end of the optical path, to produce sensing signals, and inthat the method further comprises detecting the sensing signals at oradjacent the first end of the optical path.

[0015] Preferred embodiments of the present invention will now bedescribed, by way of example only, with reference to the accompanyingdrawing, of which:

[0016]FIG. 1 shows an optically-powered hydraulic valve control systemin accordance with a first embodiment of the present invention;

[0017]FIG. 2 shows the flow connections between components of FIG. 1;

[0018]FIG. 3 shows a differential pressure flow sensor for use insystems according to the present invention;

[0019]FIG. 4 shows a power-by-light hydraulic pilot valve arrangementfor use in systems according to the present invention;

[0020]FIG. 5 shows an optically-powered actuator system in accordancewith a second embodiment of the present invention;

[0021]FIG. 6 shows a modification of the system of FIG. 5;

[0022]FIG. 7 shows optical spectra relating to the system of FIG. 5;

[0023]FIG. 8 shows a strain signal processing scheme block diagramrelating to the system of FIG. 5; and

[0024]FIG. 9 show signal waveforms relating to FIG. 8, with FIG. 9arepresenting raw reference data and raw scanning data, and FIG. 9brepresenting a correlation output.

[0025]FIG. 1 shows a monitoring system 10 comprising an optical powersource in the form of a laser 11 for generating light to operate apower-by-light (PBL) hydraulic valve arrangement 15. The valve is usedto control flow-rate, flow direction and/or pressure in a hydraulicsystem. A high temperature grade, low numerical aperture (NA) multimodeoptical fibre 13 connects source 11 to integrated optical fibre basedsensors 14 and, via a further optical fibre 23, to the valve 15 whichincorporates a piezoelectric actuator. Hydraulic inlet and outlet portsare indicated schematically at 18 and 19 respectively.

[0026] The sensors 14 include a flow sensor and a temperature sensor andwill be described in further detail below.

[0027]FIG. 2 shows the hydraulic connections between the sensorarrangement 14 and the valve 15. It will be noted that the ports P1 andP2 of the sensor arrangement are in communication with the hydraulicpassage 27 between inlet 18 and outlet 19. An orifice plate 20 islocated in passage 27 between ports P1 and P2.

[0028] In one preferred arrangement a 150 mW laser source emitting an810 nm peak wavelength uses a 50:50 multiple mode optical fibre coupler12. One output end of the coupler is connected to multimode opticalfibre Bragg gratings (mm-FBGs) which are used for differential pressuresensing and temperature sensing. The other end (at the input side) ofthe optical fibre coupler is connected to an optical spectrum analyser.

[0029] In another preferred arrangement the optical source 11 is an SLD5 mW, L3302-01 laser from Hamamatsu emitting an 845 nm peak wavelength.The optical power is launched into a 50:50 single mode optical fibrecoupler 12. Two output ends of the coupler are connected to opticalfibre Bragg gratings (FBGS) which are used on a diaphragm means of thevalve arrangement. Again, the other end (at the input side) of theoptical fibre coupler is connected to an optical spectrum analyser (e.g.an Anritsu model MS99A, resolution 0.1 nm).

[0030] The optical fibre 13 is used to channel the sensing informationfrom sensors 14 in the opposite direction to the power supplied bysource 11. The information is channelled by fibre optic coupler or beamsplitter 12 from the main optical fibre via a separate optical path 22into an optical signal processor and monitoring unit 16. The unit 16detects and converts the optical signal sent reflected from the sensorsin the system to electronic information. The electrical information isthen adapted and supplied to a further unit 17 including a signalprocessor, a control panel, a user interface and a display unit. Theadapted information is supplied to a control panel display. A controllink from unit 17 to the laser 11 is indicated by numeral 32.

[0031] The optical fibre sensors 14 are used for sensing the PBL valveoperation and allow most of the optical power to pass through to the PBLvalve or actuator 15. The sensors 14 comprise multimode fibre Bragggratings (MM-FBGs) to sense the hydraulic flowrate, hydraulic pressureand fluid temperature in the hydraulic PBL valve system. In particular,one MM-FBG sensor 41 for strain is mounted between two diaphragms 42,FIG. 3, to sense the differential pressure proportional to flow-ratethrough the PBL valve 15. The second MM-FBG sensor 43 is incorporated asa temperature reference and compensates the differential pressure (DP)based flow sensor. The components are mounted in a housing 44. Thesensor 43 is located so as not to be affected by movements of thediaphragms 42.

[0032] The temperature sensing MM-FBG, pressure sensing MM-FBG and PBLvalve are connected in series through the same optical powertransmission line 13, 23. Information regarding the fluid flowrate isreflected and returned up the same fibre waveguide 13 in the form ofdifferential optical wavelength shifts. To detect the information, theunit 16 employs a wavelength demodulator to process the signals. Thetemperature information is also extracted. Wavelength divisionmodulation (WDM) permits the strain and temperature signals to beextracted separately.

[0033]FIG. 4 shows an optical power valve arrangement 15 for hydraulicor pneumatic application, which is used to open and close apower-by-light valve. The arrangement comprises a piezoelectricmultilayer or bimorph bender element 151, an elastomer blocking plate152, an inlet nozzle 18, an outlet nozzle 19 and a valve case or housing154. The valve is driven by electrical power, which is converted fromoptical power by a photovoltaic converter 155 and DC-DC converter 156.The optical power output from the sensor 14 is fed into the photovoltaic converter 155 to convert into electrical power to drive element151 via electrical leads 160 which are connected to the fixed clamp end161 of element 151.

[0034] A preferred hydraulic valve arrangement 15 has an operatingpressure of greater than 950 kpa.

[0035] In the valve arrangement 15, the orifice plate 20, FIG. 2, isused to restrict the fluid flow and cause a pressure drop across itself.The thin plate diaphragms 42 are strained by applying this differentialpressure to them. In an alternative arrangement, a single FBG 41 may bemounted on a single diaphragm 42, which is arranged such that the radialstrain on the diaphragm surface is related to the applied pressure. Thestrain is transformed into optical information by using the opticalfibre Bragg grating 41 mounted on the diaphragm(s) 42. The secondgrating 43 is for temperature measurement and temperature compensation.The differential wavelength shift between the reflected signals from thetwo gratings provides a measurement of the fluid flow-rate. Temperaturevariation causes the variation in the reflected wavelengths to be thesame and therefore there is no change in differential wavelength(assuming perfect symmetry). Typical operating characteristics are afluid flow-rate between 0-800 cm³/s at a pressure of 50 Bar andtemperature range of −4° C. to 160° C., preferably 25° C. to 75° C.

[0036] To manufacture preferred sensors, Bragg gratings are fabricatedon multimode or single mode silica optical fibre. The output ends ofeach FBG sensor are connected directly to the valve to supply it withpower.

[0037] The DP sensor is connected to the differential pressure acrossthe orifice plate.

[0038] The above system is especially suitable for long transmissionlines, typically about 5 km, thus permitting remote control of the valve15. The use of wavelength modulation and demodulation is especiallysuitable for long distance applications. In addition, the system canresist harsh environments since the actuator in the valve is capable ofoperating in high temperatures. The actuator also has the advantage ofrelatively low power consumption.

[0039] The use of multimode fibre as the line 13 allows relatively highlevels of power to be transmitted, over shorter distances, whereassingle mode fibre is generally preferred for use over longer distances.

[0040] The system is self-contained in that the source 11 generateslight to operate valve 15 and all the sensors in the system. The opticalpower attenuation associated with the sensors is small in comparisonwith the power passing through to actuate the valve 15. In particular,the sensors 14 pass the optical power directly to the valve 15 withoutany coupler or splitter between them. Thus, the need is eliminated forcomplicated and unreliable optical assemblies at the remote end, ieadjacent to valve 15.

[0041] The optical control loop utilises the same fibre for the majorityof the transmission line where power and sensing signals travel inopposite directions. This control system can be operated without usingoptical fibre couplers or beam splitters at the remote end. Using asingle fibre optic transmission line reduces costs, complexity,maintenance and allows easier fibre deployment.

[0042] Variation in optical power levels to the valve 15 can be used tovary the flow rate through the valve. Variation of flow rate through thevalve 15 can be detected using this system. Power levels to the valveare controlled using pulse width modulation to the valve or by the powersetting for continuous wave emission from the light source.

[0043] Various modifications can be made to the above-describedarrangement. For example the optical source 11 may be an electricallight source or a semiconductor light source, white light, LightEmitting Diode (LED), Super Luminescent Diode (SLD), Edge-emitting LED(ELED), Laser Diode (LD) or Vertical Cavity Surface Emitting Laser Diode(VCSEL). The transmission line 13 may be multimode or single-modeoptical fibres or other forms of optical waveguide. The samespecification is used for the coupler/splitter 12 as for the opticalfibre 13. Alternatively, the beam splitter or combiner 12 may be a cubeglass beam splitter/combiner, angle glass, wavelength divisionmultiplexing/demultiplexing (WDM/DWDM) coupler, silica-based buriedchannel waveguides coupler or optical couplers/splitters.

[0044] The actuator in the valve 15 may be piezoelectric, ferroelectric,electromagnetic, photostrictive, a shape memory alloy or electrostatic.The actuator utilises the extremely small amounts of power supplied tooperate effectively.

[0045] The components of sensors 14 and actuator/valve 15 may becombined or separated as desired.

[0046] Instead of FBGs, sensors 14 may employ the techniques ofmicrobending, Fabry-Perot sensing, or Rayleigh or Brillouin scattering.

[0047] The optical monitoring and processing unit may include an opticalspectrum analyser, a CCD array grating, tuneable filter, a scanningfilter, an optical interferometer, an optical wavelength edge detectorand an optical power ratio detector. However, only a single detector maybe provided.

[0048] The electronic processing can be microprocessor based ormicro-controller based. Software can also be adapted for closed-loopfeedback control of the light source 11.

[0049] The optically powered actuator system of FIG. 1 may be modifiedto employ more than one light source for distributed sensors. Thus FIG.5 shows a monitoring system 110 with a laser source 11 and a widerbandwidth super luminescent diode 111 for the flow and temperaturesensors. A 1×Z bidirectional fibre optic coupler 12 is employed.

[0050] The second optical source 111 can be used to monitor when theoptical fibre is broken. The laser source 11 is interlocked with thesecond source monitoring arrangement so that the laser is turned off forsafety reasons if a broken fibre is detected.

[0051] Again, the optical fibre 13 in the system can utilise eithermultimode or single mode optical fibre. A multimode fibre is the mostsuitable for the short distance system (<5 km), because it is easy tocouple to optical sources and inexpensive components. A single modeoptical fibre is preferred when the system distance is over 5 km,because the single mode fibre has a very low attenuation compared to themultimode fibre.

[0052] Also, as in the embodiment of FIG. 1, an optical fibre based flowsensor or flow switch 14 of FIG. 5 employs wavelength-based modulationand it allows the optical power to pass into the actuator unit of valve15. For example, a differential pressure flow sensor 14 can use anoptical fibre Bragg grating in either single mode or multimode.

[0053] The optical signals from both light sources 11, 111 aretransmitted into the sensor 14. The laser power passing through the FBGsensors 14 into the actuator and the optical power from second lightsource 111 is reflected back by the FBGs. The reflected signal from theFBGs is wavelength modulated and is transmitted along the same opticalfibre 13 and is fed into signal monitoring unit 16 and processor unit17. Whereas prior art techniques, such as that disclosed in U.S. Pat.No. 5,848,204 use multimode FBGs in extrinsic mode for strainmeasurement based upon microbending and mode losses, the presenttechnique is based upon intrinsic multimode FBGs, which offer simplerdesign, lower cost and better performance. Use in the “extrinsic” modemeans use for reflection only, whereas use in the “intrinsic” mode meansuse for sensing in addition to reflection.

[0054]FIG. 6 shows a modified monitoring system 210 in which, by usingan optical coupler 212, the actuator arrangement may comprise amultiplexed plurality of sensor/valve combinations 14, 15.

[0055]FIG. 7 indicates the spectra of the input and output signals toand from the sensors 14. With a flow passing through the orifice plate20, a differential pressure is produced and the MM-FBG 41 locatedbetween the diaphragms 42 is strained. The FBG reflected signalwavelength is shifted proportional to the applied strain. The amount ofwavelength shift is related to the applied pressure and flow rate andcan be monitored and processed by units 16 and 17.

[0056] Various monitoring schemes can be used by the optical signalprocessor and monitoring unit 16 and 17. The signal processing scheme inunit 17 could be: using the centroid of the MM-FBG spectra, fuzzy logic,artificial neural networks, and/or correlation. For example,auto-correlation and cross-correlation can be used to analyse the datafrom the MM-FBG 41 and they can be completed in less than 1 second. Thecross-correlation, r₁₂(n) between two data sequences, each containing Ndata points, can be written as:${r_{12}(n)} = {\frac{1}{N}{\sum\limits_{n = 0}^{N - 1}{{x_{1}(n)}{x_{2}(n)}}}}$

[0057] where x₁(n) is the first data sequence and x₂(n) the second datasequence.

[0058] For the strain measurement the MM-FBG spectra (scanned by anoptical spectrum analyser) at zero strain are used to provide areference pattern ε_(ref). The raw reference data is stored in acomputer memory and is processed by the auto-correlation method. Theoutput from the auto-correlation is used as the correlation strainreference Cε_(ref), and this is also saved in computer memory. The newscanning data is processed again after the strain has been varied. Theapplied strain scanning data ε_(scan) is compared with the correlationstrain reference Cε_(ref). The distance between maximum points of eachresult, Cε_(scan)(n_(max))−Cε_(ref)(n_(max)) (where n_(max) is the dataindex point at maximum value of a correlation result), can be convertedinto MM-FBG wavelength shift information. A signal processing blockdiagram is shown in FIG. 8.

[0059] As can be seen in FIG. 9, the reference signal is scanned andsaved in the PC memory and then the raw signal is measured (FIG. 9a). Inorder to establish the amount of wavelength shift, correlation signalprocessing (FIG. 9b) is used to evaluate the maximum correlation pointsbetween reference and measured signals. The difference of the positionof the highest peaks form the two output signals obtained from thecorrelation process is then converted back to wavelength shift ofreflection spectra.

[0060] The features and modifications of the embodiments of FIGS. 1 and5 may be interchanged as desired. The second source 111 may be any ofthe types of light source given above.

[0061] The arrangement disclosed in FIG. 5 allows the use of two sensorsin series and greater sensitivity. The use of more than one lightsources increases the optical bandwidth and hence improves theresolution of the or each sensor or allows an increase in the number ofsensors used, or both increases resolution and the number of sensorsthat can be used.

[0062] The use of a 1×Z coupler 12 (with Z greater than or equal to 3)allows greater sensitivity, use of more sensors and/or interlock withina single control system. The use of a 1×Z coupler also allows the use ofmany actuators and/or valves at the remote end (15).

[0063] Instead of a hydraulic valve, the above-described systems can beused in conjunction with a wide range of control elements in pneumatic,gas powered or other control systems. Typical applications include theaircraft, petrochemical, oil and gas exploration industries and use infuel system applications. The advantages of arrangements according tothe present invention in these contexts include intrinsic safety inhazardous environments, low attenuation, immunity from electromagneticinterference, cable weight and ease of deployment. Moreover,arrangements according to the present invention provide feedback-sensinginformation using the same optical power transmission fibre withoutusing any coupler or beam splitter at the remote harsh environment end.

[0064] Instead of being both powered and controlled optically, theactuator may be powered or controlled optically with the other functionbeing effected in some other way, e.g. electrically, by electromagneticinduction, radio waves etc.

[0065] The term “optical” as used herein embraces infra-red,ultra-violet and other non-visible electromagnetic wavelengths.

1. A monitoring system (10, 110, 210), for an optically powered and/orcontrolled actuator arrangement (14, 15) comprising an optical path (13)arranged to be connected between at least one optical source (11, 111)and the actuator arrangement for supplying power and/or controlinformation thereto, the actuator arrangement including means forconverting the optical power and/or control information, characterisedin that the actuator arrangement (14, 15) further comprises means (14)for sensing the operation thereof and providing an optical data outputback through said optical path (13) and in that means (16, 17) areconnected to the optical path to monitor the output of said sensingmeans (14).
 2. A monitoring system according to claim 1, wherein thesensing means (14) is also operated by the optical source (11).
 3. Amonitoring system according to claim 1 or 2, wherein the sensing means(14) comprises one or more fibre Bragg gratings (41, 43).
 4. Amonitoring system according to claim 3, wherein a differential pressureof the actuator arrangement is sensed by an arrangement incorporating afibre Bragg grating (41) located adjacent to at least one diaphragm(42).
 5. A monitoring system according to claim 4, wherein the fibreBragg grating (41) is mounted between two diaphragms (42) arranged to beexposed to different pressures.
 6. A monitoring system according to anyof claims 3 to 5, wherein at least one fibre Bragg grating (43) isprovided for sensing the temperature.
 7. A monitoring system accordingto any preceding claim comprising at least two optical sources (11,111), a first (11) of which is used for supplying power and/or controlinformation, and a second (111) of which is used to detect interruptionof the optical path (13) and is arranged to turn off the first source(11) in the event of such interruption.
 8. A monitoring system accordingto claim 7, wherein the first optical source (11) is a laser.
 9. Amonitoring system according to any preceding claim wherein themonitoring means (16, 17) includes an optical spectrum analyser.
 10. Amonitoring system according to any preceding claim comprising an opticalcoupler or beam splitter (12) located in the optical path.
 11. Amonitoring system according to any preceding claim, wherein the outputof the sources (11, 111) is controlled by a signal (32) from themonitoring means (16, 17).
 12. A monitoring system according to anypreceding claim, wherein the actuator arrangement (14, 15) is at alocation remote from the optical source (11, 111).
 13. A monitoringsystem according to any preceding claim, wherein the actuatorarrangement (15, 15) is multiplexed (FIG. 6).
 14. A method of using oneor more light sources (11, 111) at a first end of an optical path (13)for powering and/or controlling a valve or actuator (15) at a second endof the optical path characterised in that light from the source(s) (11,111) is used to interact with one or more sensors (14), at or adjacentthe second end of the optical path, to produce sensing signals, and inthat the method further comprises detecting the sensing signals at oradjacent the first end of the optical path.
 15. A detection device (14)comprising at least one fibre Bragg grating (41) for detecting movementof an adjacent diaphragm means (42) characterised in that the diaphragmmeans comprises two generally parallel diaphragms (42) and the grating(41) is located therebetween.